In recent years, with high performance tendency of automobiles, tires are required to have high steering stability and enhanced riding comfort, and it is required to reduce weight of tires in view of resource conservation concerning environmental problem.
On the other hand, in the case of a pneumatic tire, especially pneumatic radial tire, it is known that if lateral rigidity of the tire is increased, the steering stability can be enhanced. For this reason, in order to increase bending rigidity of a sidewall portion of the tire, there exists a so-called 1—1 structure in which a carcass comprises two carcass plies, one of the carcass plies is wound up from inside to outside of a bead core in an axial direction of the tire, and a lower portion of the other carcass ply is wound down outside of a bead apex in the tire axial direction.
According to this structure, since the two carcass plies sandwich the bead apex, when bending deformation is applied to the bead portion of the tire by stress in the tire axial direction, compressive force is applied to one of the carcass plies and tensile force is applied to the other carcass ply. Therefore, a neutral line of bending is located in the bead apex which is made of hard rubber, and lateral bending rigidity is increased, and the steering stability is enhanced.
However, in the outer carcass ply in the tire axial direction which was wound down, conventionally, when a lower end edge of the wound down portion is located at a tire radially outer side of the bead core, the lower portion is located outside of the wound up portion of the first carcass ply in the tire axial direction, or between the wound up portion and the bead apex.
In such a structure, however, the lower end edge of the wound down carcass ply is prone to be close to a free end, and the lower end edge can not sufficiently endure the tensile force when bending deformation is applied to the bead portion, and the carcass ply can not contribute to enhancement of lateral rigidity of the tire as compared with a carcass ply which is retained by the bead core.
If the wound down carcass ply is wound from tire radially inward of the bead core to inner side in the tire axial direction and is retained, and a fixing strength at the bead portion is enhanced, it is possible to enhance the lateral rigidity, but assembling of parts when tire is formed becomes difficult, productivity is lowered, and since the length of the carcass ply is increased, a tire weight is also increased.
It is a well known technique to dispose, in the bead portion, a reinforcing layer made of cord material having high rigidity such as metal cord, thereby enhancing the bending rigidity of the tire, but increase in tire weight can not be avoided, riding comfort is deteriorated in generally, and there are many tires which do not have such reinforcing layer.
Japanese Patent Application Laid-open No. H6-344711 proposes a structure in which a wound down carcass ply is folded outside of the bead apex in the tire axial direction, thereby enhancing the rigidity of the bead portion. According to this proposed structure, the tire can be reduced in weight as compared with a tire using the reinforcing layer, the sandwiching pressure degree can be increased at the bead portion by the folding and the fixing strength can be enhanced to some degree, but it is desired to further enhance the fixing strength.